code: def forward(self, **kwargs):
      self.output = self.nn(self.pos)
      self.loss = self.output.sum()
  
file_path: torch-points3d/test/mock_models.py
imports: from torch_points3d.models.base_model import BaseModel
  from torch import nn
label_desc: Use image node_id 3 for calling the DifferentiableMockModel obj's underlying member method code with example usage: obj.forward() without return types